Given the way the autosave feature was awkwardly bolted on to the
composer, an EExtension seemed like a natural fit.  And it helped
clean up some object lifecycle hacks (and bugs).

What we have now is a new module consisting of two EExtensions:

   EComposerAutosave extends EMsgComposer and determines when to
   kick off an asynchronous autosave operation.

   EComposerRegistry extends EShell and offers to restore orphaned
   autosave files on startup (which is also asynchronous now).

e-autosave-utils.c holds the actual asynchronous functions and a few
other miscellaneous utility functions.

Source code for the new module lives in /modules/composer-autosave.